A. Technical Field
The present invention relates to a microelectromechanical structure, and more particularly, to systems, devices and methods of compensating effect of thermo-mechanical stress by incorporating and adjusting elastic elements that are used to couple a moveable proof mass and anchors included in the microelectromechanical structure.
B. Background of the Invention
A microelectromechanical structure is widely applied as a sensor to measure acceleration, rotation, pressure, temperature and many other physical parameters. The microelectromechanical structure is normally formed on a silicon substrate using a micromachining process that results in characteristic feature sizes of several micrometers. Such miniaturized devices may transduce mechanical movement to electrical properties. One typical microelectromechanical structure is a micromachined capacitive accelerometer that comprises a proof mass that is suspended above a silicon substrate.
FIG. 1A illustrates a z-axis capacitive accelerometer 100, and FIG. 1B illustrates its response 150 to an out-of-plane acceleration. The z-axis capacitive accelerometer 100 has its moveable proof mass 102 anchored to a substrate 104 via an anchor 106. Electrodes are formed on both the proof mass 102 and the stationary substrate, such that two capacitors 108A and 108B are formed and arranged on two sides of the anchor 106. In response to an out-of-plane acceleration along the z-axis, the proof mass 102 tilts with respect to its rotation axis 106. The gaps between the proof mass 102 and the corresponding electrodes, 110A and 110B, vary differently, and lead to a mismatch between capacitors 108A and 108B. Such a capacitance mismatch is processed by an interface circuit to output a signal that indicates the magnitude of the out-of-plane acceleration.
Thermo-mechanical stress may introduce an intrinsic offset or drift to the output even though no out-of-plane acceleration is applied. In an ideal situation, the capacitance mismatch between capacitors 108A and 108B should only be associated with the out-of-plane acceleration, and does not exist when no out-of-plane acceleration is involved. However, thermo-mechanical stress may be accumulated in the substrate 104 during the course of soldering and packaging, unavoidably causing the substrate to warp. When such warping is symmetric with respect to the anchor 106, the capacitor gap variations happen to cancel out and result in no capacitance mismatch between capacitors 108A and 108B. In most cases, substrate warping is asymmetric. Regardless of how close they could be, gap variations of capacitors 108A and 108B are always different. Such asymmetric gap variations are incorporated into the final output from the interface circuit, and lead to an offset in the sensed acceleration and a sensitivity drift for the accelerometer 100.
Apparently, device performance of the capacitive accelerometer 100 is compromised due to the thermo-mechanical stress in the substrate 104. Such performance degradation is commonly shared by microelectromechanical structures that rely on suspended proof masses for sensing and transducing mechanical movement. A need exists to compensate the thermo-mechanical stress that is built up in the substrate 104 during the course of manufacturing, packaging, assembly and lifetime operation.